The process of xerography, as disclosed by Carlson in U.S. Pat. No. 2,297,691, employs an electrophotographic element comprising a support material bearing a coating of an insulating material whose electrical resistance varies with the amount of incident electromagnetic radiation it receives during an imagewise exposure. The element, commonly termed a photoconductive element, is first given a uniform surface charge, generally in the dark after a suitable period of dark adaptation. It is then exposed to a pattern of activating radiation, such as visible light or X-rays, which has the effect of differentially reducing the potential of the surface charge in accordance with the relative energy contained in various parts of the radiation pattern. The differential surface charge or electrostatic latent image remaining on the electrophotographic element is then made visible by contacting the surface with a suitable electroscopic marking material. Such marking material or toner, whether contained in an insulating liquid or on a dry carrier, can be deposited on the exposed surface in accordance with either the charge pattern or discharge pattern as desired. Deposited marking material can then be either permanently fixed to the surface of the sensitive element by known means such as heat, pressure, solvent vapor or the like, or transferred to a second element to which it can similarly be fixed. Likewise, the electrostatic charge pattern can be transferred to a second element and developed there.
Various photoconductive insulating materials have been employed in the manufacture of electrophotographic elements. For example, vapors of selenium and vapors of selenium alloys deposited on a suitable support and particles of photoconductive zinc oxide held in a resinous, film-forming binder have found wide application in present-day electrophotographic document copying processes.
Since the introduction of electrophotography, a great many organic compounds have also been screened for their photoconductive properties. As a result, a large number of organic compounds have been known to possess some degree of photoconductivity. Many organic compounds have revealed a useful level of photoconduction and have been incorporated into photoconductive compositions. Among the various reasons for the increasing interest in the investigation of organic materials as photoconductors for typical photoconductive elements used in electrophotographic processes is that many of these materials are optically clear in addition to having desirable electrophotographic properties. Therefore, such materials can be used as a transparent coating adhered to a suitable support in an electrophotographic apparatus. Because of this transparency property of many organic photoconductive materials one attains additional flexibility in equipment design, i.e., one has the option of exposing such transparent materials from either the top surface of the material coated on a suitable support or, if the support is also transparent, one can expose through the support onto the bottom surface of the material.
One particular organic photoconductive composition which has received considerable interest in the art is an organic photoconductive composition composed of a charge transfer complex consisting of approximately equal molar amounts of a polymerized vinyl carbazole compound and a material which is an electron acceptor for the vinyl carbazole compound, such as 2,4,7-trinitro-9-fluorenone (often referred to in the art and hereafter in the present application as TNF). Further description of this particular photoconductive material and various background patent literature relating thereto may be found in Shattuck et al U.S. Pat. No. 3,484,237 issued Dec. 16, 1969; Hoegl U.S. Pat. No. 3,037,861 and Hoegl Canadian Pat. No. 690,972 issued July 21, 1964.
The materials described in the foregoing patent literature have received extensive attention and investigation, particularly the photoconductive compositions composed of a mixture of polyvinyl carbazole and TNF, and have been used in commercial electrophotographic office-copier equipment. In addition, much work has been carried out to further improve, modify and, in fact, replace one or both of the materials used in such charge-transfer complex photoconductive compositions to obtain improvements in the performance of these compositions in electrophotographic imaging processes. In particular, such work has been done to find and develop other types of photoconductive charge-transfer compositions which exhibit increased sensitivity to activating radiation so that the resultant photoconductive composition can be used in higher speed electrophotographic copy duplicating equipment or can be used in conventional speed electrophotographic equipment together with less intense exposure sources.
In this regard, Contois et al U.S. Pat. No. 3,655,378 issued Apr. 8, 1972 describes a photoconductive composition composed of a charge-transfer complex of a Lewis acid, such as TNF and, instead of a polyvinyl carbazole resin, a resin formed by the condensation of dibenzothiophene with formaldehyde or a resin formed by the condensation of formaldehyde and dibenzofuran. Organic photoconductive compositions containing such charge-transfer complexes exhibit useful levels of light-sensitivity generally equal or comparable to that obtained by charge-transfer complex photoconductive compositions composed of polyvinyl carbazole and TNF. In addition, it was found that the sensitivity of the resultant charge-transfer complex organic photoconductive compositions described in U.S. Pat. No. 3,655,378 could be enhanced by the addition of relatively small amount of various sensitizing dyes and/or chemical sensitizers, including certain materials previously known in the art as organic photoconductors, for example, tris(p-tolyl)amine.
Although the various organic charge-transfer complex photoconductive compositions such as those described in Contois et al U.S. Pat. No. 3,655,378 and Shattuck et al U.S. Pat. No. 3,484,237, referenced hereinabove have been found quite useful, further research activity has gone on in the art to find even further improvements and modifications of such materials. In particular, extensive efforst has been devoted to find compositions exhibiting increased light sensitivity so that certain charge-transfer complex containing compositions can be used in higher speed electrophotographic processes.